This proposal is for renewal of a five year grant which terminated April, 1978, to study the biomechanical environment of the hip. The major original objective of that grant was to improve the surgical treatment of patients with arthritic hips by estimating the three-dimensional loading on the hip and then identifying ways to reduce that loading through surgical geometry and implant design. A sophisticated, reliable method to estimate the three-dimensional loading of the hip during gait and activities of daily living has been developed. The method involves the use of biplanar photography to record body segment displacement histories. Foot-floor reactions are measured with a force plate. Body segment inertial properties are determined with anthropometric measurements. The kinematic, kinetic, and inertial property data is combined in a Newtonian model to predict the intersegmental resultant forces and moments. The resultants are then "distributed" to the muscles and articular surfaces of the hip using a linear optimization scheme. This method has been applied to study loading in the human hip and to identify ways to decrease loads and stresses with treatment. We propose continuing our studies of the hip and extending them to the knee joint. We will develop a refined three-dimensional model of the lower extremity muscle and ligament anatomy. We will then investigate new schemes to distribute the resultant hip and knee forces and moments to the muscles and ligaments. Validation procedures on the various distribution schemes will be carried out for normal and pathologic gaits and other activities of daily living. Those schemes which are most appropriate will then be used to collect data on the mechanical function of normal and diseased hips and knees. This data will be analyzed in forms useful for designing treatment modalities.